1. Field of the Invention
This invention is directed to a windback device with a rotating shaft and positioned between a lubricant sump and a shaft seal so as to prevent lubrication fluid from reaching the seal.
2. Description of the Prior Art
With rotating shafts, lubricant sumps are frequently required along segments of the shaft to provide lubrication for shaft bearings and the like. It is frequently desirable to isolate the lubricant sump from remaining media in which the shaft is immersed, as well as to assure that the lubricant remains in the sump so that (1) lubrication of the bearings may continue and (2) the lubricant does not contaminate any other media in which the shaft is immersed.
For example, in gas turbine applications the lubricant sump is typically isolated from the remainder of the gas turbine housing by the sump housing and a shaft seal. The remainder of the gas turbine housing, or a compartment adjacent thereto, is typically filled with gases at temperatures and pressures higher than those desired in the lubricant sump. It is often important to prevent lubricant from mixing with gas contained in such high temperature gas and/or pressurized compartments. For example, when an oil lubricant is used, mixing the oil with the gas could result in formation of oil coke, which is a byproduct of oil heated to an elevated temperature and which chemically alters the oil and hence is detrimental to the gas turbine. Oil coke can foul seal surfaces and prevent proper bearing lubrication. Accordingly, it is important in many applications that the lubricant be isolated within a lubricant sump.
To accomplish such sump isolation, a shaft seal is normally positioned adjacent to the sump and around a rotatable shaft passing therethrough. While the sump is typically at a lower pressure than the remainder of the turbine, research has shown that pressure differentials within the sump are such that lubricants held within the sump are encouraged to travel along the rotating shaft toward the seal. These pressure differentials, along with shaft windage, tend to move the lubricant out of the sump and along the shaft, a phenomenon often termed “lubricant splash,” which results in loss of lubricant from the lubricant sump.
Some sealing means, such as the shaft seal, are normally provided to at least minimize if not prevent lubricant splash and lubricant loss. However, the shaft seal, which typically is a segmented carbon rubbing seal, may have the carbon sealing faces contaminated by the lubricant if the lubricant reaches the seal. Such contamination of the seal can compromise the integrity of the seal and reduce the efficiency with which the seal isolates the lubricant within the lubricant sump.
To prevent lubricant splash from reaching the carbon sealing faces or other parts of the seal, a windback device is typically interposed along the shaft, between the seal and the lubrication sump. The windback device normally has an annular collar-like member, receiving the shaft in the opening of the collar-like member, and a screw pitch or thread in the radially inward surface of the collar-like member that faces the shaft. The windback device is typically secured to the housing to prevent lubricant leakage from the sump to the remainder of the housing except along the shaft. The shaft seal is normally interposed between the windback device and the shaft, with the seal located along the shaft, on the side of the windback device that is remote from the sump in order to prevent lubricant leakage along the shaft.
Since a windback device is typically in close proximity to the associated shaft, yet not so close as to expect to create a pressure drop along the shaft, initial conventional wisdom was that pressure along the opening in the windback device, namely at positions in the windback device opening that were adjacent to the rotating shaft, would be essentially constant along the length of the shaft within the windback device. As a result of this assumption, conventional wisdom was that any lubricant splash moving along the shaft of the windback device would likely flow back toward the sump along the threaded inner surface of the windback collar.
A number of pressure measurements taken along the axial length of the windback device showed the surprising result that a pressure drop actually existed between the sump and the end of the windback device remote from the sump. Measured pressure at the sump end of the windback opening was greater than measured pressure at the seal end of the windback opening. Thus, lubricant was encouraged, by this pressure different in an unexpected direction, to move away from the sump along the shaft to other interior portions of the shaft housing, responsively to the pressure drop along the windback in that direction.
In applications such as gas turbines, the pressure of the lubricant medium in regions other than in the sump is higher than the pressure within the sump. Thus, one would expect that the lubricant pressure in the sump, adjacent to the seal, would be higher than the lubricant pressure in the remainder of the sump. However experiments have shown that, even in such applications, the pressure in the sump adjacent the seal was lower than the pressure in the remainder of the sump, thus causing additional lubricant flow along the shaft towards the seal. Because of this unexpected pressure profile, windback devices prior to the aforementioned research failed to fully prevent the flow of lubricant toward the seal.
U.S. Pat. No. 5,503,407 addressed the aforementioned research discloses one solution to this problem. Specifically, referring to FIG. 1 of this application, which has been adapted from a similar figure in '407, '407 discloses a windback as part of a circumferential seal assembly within a lubricant sump. The seal assembly, specifically the windback, has an internal helical thread (A) machined on the bore of the housing, wherein, a shaft passes through and rotates within the bore. When the shaft rotates, windage results in the clearance between the windback and the shaft. Lubricant droplets entering between the windback and shaft clearance from the lubricant side are, thereby, forced on the helical thread (A) due to the effect of windage drag. The lubricant droplets are further forced by the shearing stress on the thread surface to move circumferentially along the helical thread (A) to a single windback bleed-off slot (B) wherein the lubricant is returned to the lubricant sump.
This helical thread windback design of the '407 patent causes the lubricant droplets to travel several times around the circumferential distance deferred by the helical thread (A) of the windback to the singular leak-off slot (B) before dropping into the lubricant side. Ultimately, the circumferential distance around the windback helical thread (A) combined with only one leak-off slot decreases the ability of the helical thread of the windback '407 to efficiently pump the lubricant back into the sump. The additional distance and single leak off slot also increases the risk that the windback will clog or be backed up with lubricant, reducing efficiency of the helical thread (A).
An alternate, earlier solution is presented by U.S. Pat. No. 5,322,298. More specifically, '298 discloses a windback with first and second annular grooves. The annular grooves are perpendicular to the plane of the shaft and are spaced such that the first annular groove defines a distal end of the windback relative to the lubricant sump and the second annular groove is at a proximal end of the windback relative to the lubricant sump. Both annular grooves substantially surround the shaft. Threads extend along the windback bore at an oblique angle, relative to the longitudinal axis of the shaft, between the first and second annular grooves. In operation, lubricant drops entering between the windback and the shaft are deposited into the oblique threads by the windage drag. The shearing stress caused by the rotation of the shaft forces the fluid to move along the oblique threads and into the annular grooves. However, there is no leak-off slot to release build up of fluid from the annular grooves. To this end, in '298 fluid can build within the grooves, leading to a decrease in the windage pumping efficiency and reintroduction of lubricant into the space between the shaft and the windage.
Accordingly, a windback device is desirable that can efficiently redirect fluid entering the space between a windback and a shaft back into the sump and that can redirect the fluid flow, while reducing build-up of the fluid within the windback.